Not applicable.
The present invention relates to methods for reducing the leaching potential of environmental chromium-contaminated particulate matter to acceptable levels. More particularly, this invention describes methods for stabilizing hexavalent chromium in chromium-contaminated materials.
In the environment, chromium exists predominantly in two formsxe2x80x94hexavalent chromium and trivalent chromium. Trivalent chromium is significantly more stable than hexavalent chromium, which is highly mobile. It is known that a near-neutral pH is required to keep trivalent chromium in a stable, insoluble state. Hexavalent chromium is a known human carcinogen, a RCRA hazardous material, and a common contaminant on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) Priority List of Hazardous Substances. Although chromium metal, like other metals, exhibits a positive valence state, hexavalent chromium is typically present in the environment as an oxy-anion such as chromate (CrO42xe2x88x92) or dichromate (Cr2O72xe2x88x92). As a result, technologies that effectively treat other cationic metals do not effectively stabilize chromium. The oxidation state of chromium, the oxidation-reduction potential (ORP) and the pH of the waste material are key parameters for controlling the leaching potential of chromium in the environment.
Reducing agents such as ferrous sulfate can stabilize hexavalent chromium in some environments. However, in alkaline environments, ordinary ferrous sulfate treatment of certain chromium-bearing wastes, including chromium ore processing residue (COPR), reduces chromium leachability for only a short time after treatment. Over time, the pH of the waste matrix slowly rises due to the high alkaline (e.g., lime) content, and chromium again leaches from the waste.
Other known approaches for stabilizing alkaline COPR have included reduction with manganese nitrate, lactic acid, steel wool, or hardwood tree leaf litter (James, B. R., Hexavalent Chromium Solubility and Reduction in Alkaline Soils Enriched with Chromite Ore Processing Residue, J. Environ. Qual. 22:227-233 (1994)), reduction with sulfide ions (U.S. Pat. No. 3,981,965), adjustment of pH followed by the addition of organic material (U.S. Pat. No. 5,562,588), mixing with mud or dredged sediment and ground blast furnace slag (U.S. Pat. No. 4,504,321), in situ treatment with the addition of ferrous sulfate (U.S. Pat. No. 5,202,033), treatment with ferrous iron followed by the addition of silicate solution (U.S. Pat. No. 5,285,000), and treatment with ascorbic acid (U.S. Patent No. 5,951,457).
It is known that simultaneous combination of a pH control agent and an agent for controlling oxidation-reduction potential (ORP) with chromium-contaminated waste can reduce hexavalent chromium to the less mobile trivalent state. A combination of sulfuric acid for pH control and ferrous sulfate for ORP control has been used for this purpose, as have hydrochloric acid and iron chloride, respectively. A convenient source of these agents is xe2x80x9cpickle liquorxe2x80x9d from sulfuric acid-based or hydrochloric acid-based metal processing/finishing operations. Alternatively, FeSO4 and H2SO4 are known to be generated by controlled oxidation of iron pyrite (FeS2).
Cost-effective, permanent stabilizing of alkaline chromium-contaminated particulate matter has not been achieved, in part as a result of a need to convey large quantities of reactants to a stabilization site.
The present invention is summarized in that an improved process for stabilizing chromium-contaminated particulate matter and reducing chromium leaching to acceptable levels integrates previously separate processes for (1) preparing the ferrous sulfate and sulfuric acid reactants and (2) using byproducts of the stabilization process to produce additional reactants, thereby reducing or eliminating dependence upon external reactant sources.
A major objective of the invention is cost-effective, long-term, permanent stabilization of chromium in a chromium-contaminated waste matrix characterized by high concentrations of alkaline material (such as lime), where the waste can contain both trivalent and hexavalent chromium.
In the integrated process of the present invention, ferrous sulfate and sulfuric acid are used to stabilize hexavalent chromium in an alkaline environment. Iron pyrite is oxidized to produce the ferrous sulfate and sulfuric acid. The iron pyrite can be oxidized by treating it with soluble ferric ions, preferably provided as a ferric salt, such as ferric sulfate, which is itself produced as a byproduct of a reduction process wherein ferrous sulfate is used to reduce hexavalent chromium to trivalent chromium. Other pyrite oxidation means are also disclosed.
The invention will be understood by referring to the detailed description of the invention, read in conjunction with the accompanying drawings.